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Mercury exposure during pregnancy and ADHD risk in offspring

Robin Wulffson MD's picture
seafood, fish, mercury exposure, attention deficit hyperactivity disorder, ADHD

Most women exercise caution on what they consume during pregnancy. Mercury has been noted to be present in fish and the element has been reported to have health risks. A new study examined the risk of attention deficit hyperactivity disorder (ADHD) in the offspring of women who consumed fish and were exposed to mercury during their pregnancy. Researchers affiliated with Brigham and Women's Hospital and Harvard Medical School in Boston, Massachusetts published their findings online on October 8 in the Archives of Pediatrics & Adolescent Medicine.

The researchers conducted a prospective (forward-looking) study of children in New Bedford, Massachusetts from 1993 through 1998). The evaluated data from children examined at eight years of age with maternal hair mercury levels measured at the time of birth (421 children) or maternal report of fish consumption during pregnancy (515 children). They assessed inattentive and impulsive/hyperactive behaviors via a teacher rating scale and neuropsychological testing. Soon after birth, maternal hair samples were collected and analyzed for mercury levels. The mothers also filled out a questionnaire regarding fish consumption during their pregnancy.

The investigators found that the average maternal hair mercury level was 0.45 micrograms/gram (range: 0.03-5.14 μg/g); in addition, 52% of the mothers consumed more than two fish servings weekly. They found that mercury exposure was associated with inattention and impulsivity/hyperactivity; some outcomes had an apparent threshold with associations at 1 μg/g or greater of mercury. For example, at 1 μg/g or greater, the adjusted risk ratio for mild/markedly atypical inattentive behavior was 1.4 and that for impulsive/hyperactive behaviors was 1.7. (95% CI, 1.2-2.4). For neuropsychological assessments, mercury and behavior associations were detected primarily for boys. Surprisingly, however, they found a protective association for fish consumption (more than two servings per week) with ADHD-related behaviors, particularly impulsive/hyperactive behaviors (relative risk: 0.4).

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The researchers concluded that low-level prenatal mercury exposure is associated with a greater risk of ADHD-related behaviors, and fish consumption during pregnancy is protective of these behaviors. They noted that their findings underscore the difficulties of balancing the benefits of fish intake with the detriments of low-level mercury exposure in developing dietary recommendations in pregnancy.

Take home message:
Despite the finding that increased fish consumption appeared to have a protective effect from ADHD risk, it would be prudent to avoid it during pregnancy. Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, which is a highly toxic organic mercury compound. Seafood has been reported to contain varying amounts of heavy metals, particularly mercury and fat-soluble pollutants from water pollution. Species of fish that are long-lived and high on the food chain, such as marlin, tuna, shark, swordfish, king mackerel, tilefish, northern pike, and lake trout, contain higher concentrations of mercury than others. Thus, seafood containing mercury is a health hazard, particularly for women who are or may become pregnant, nursing mothers, and young children. Seafood not contaminated with mercury is a very healthy food product. Possibly, farm-raised seafood can minimize the mercury content in seafood. This would require close monitoring of the environment of farm-raised seafood.

Reference: Archives of Pediatrics & Adolescent Medicine

See also:
TV watching linked to aggression and inattention in children
Are you aware of ADHD?
Video games under development to treat ADHD